Susan has been spending five nights studying for her economics tests and her current grade in the course is 80 percent. She decides to take a part-time job and study only four nights for her next test. As a result, after her next test, her grade falls to 75 percent. The marginal cost to Susan of working one night a week is:
A. Greater than 75 percentage points and less than 80 percentage points
B. 80 percentage points
C. 5 percentage points
D. 75 percentage points